It is a common problem in buildings that high humidity occurs as a direct result of the inability of the building's air conditioning system to remove the humidity from the space. Additionally, there are many facilities with air conditioning systems that do not provide sufficient outside air ventilation to relieve the common problems of carbon dioxide buildup, airborne pollutants, and stale odors that frequently occur. When combined in a building, these conditions can result in problems such as mold, mildew, spores, dust mites, high carbon dioxide concentrations, and the presence of various other noxious or undesirable gases and odors, all of which can create health problems for the building's occupants. Recent media attention to facilities with these problems has led to their being classified as "sick buildings."
These "sick building" problems can be further exacerbated by certain general design criteria implemented by the air conditioning industry. For example, most modern air conditioning systems are designed to provide very high efficiencies in accordance with ever-increasing demands for operating cost savings. However, these high efficiencies are often obtained at the expense of the ability of the air-conditioning system to achieve moisture removal from the air, resulting in high humidity levels in the building. As a result of this type of manufacturing design philosophy, the occupants of a building having such a system will often lower the temperature setting of the thermostat in an effort to reduce the humidity in the space. This causes the air conditioning system to run for longer periods of time, increasing the operating cost of the air conditioning system and resulting in the temperature in the space becoming very cold. The net effects are a very cold, clammy (i.e., high humidity) building space having high operating costs.
One conventional solution to this problem is the use of reheat, that is, the addition of heat to the air after it has been cooled. This is done in an attempt to prevent the space from being overcooled and to obtain better control over the humidity. A negative aspect of reheat, however, is that it causes high operating costs because not only must the space load be cooled, but also the reheat load must be cooled, in effect increasing the total cooling load and the power consumed to meet this cooling requirement.
There have been many different approaches to resolving these problems utilizing energy recovery techniques, passive heat transfer, and even different coil arrangements in standard air conditioning equipment. One such example is shown in U.S. Pat. No. 4,557,116 issued to Kittler, which teaches a dehumidifier system for indoor swimming pools using heat recovery of the refrigeration compressor heat, recovered by positioning the condenser coil in the discharge air stream and reheating the air cooled in the dehumidification coil in an effort to provide reheat without additional operating costs. This approach works very well for enclosed pool environments because the discharge air temperature is approximately 100.degree. F. However, because the air is so warm, this system is unlikely to work efficiently in comfort air conditioning applications because of the large heat load resulting from introducing 100.degree. F. air into the air conditioning system. Additionally, Kittler's unit is not designed to provide any outside air ventilation cooling to the space, as it only addresses the humidity control aspect and not the fresh air ventilation or the temperature control of the air supplied to the space.
Another technique disclosed by Dinh, in U.S. Pat. No. 4,607,498, employs a heat pipe phase-change heat exchanger in combination with an evaporator coil, with a precooling coil and a reheat coil placed immediately upstream and downstream of a conventional vapor compression cycle air conditioner evaporator coil, respectively. This system can be a very effective retrofit application for dehumidification. The addition of a heat pipe heat exchanger to an existing evaporator coil has the effect of cooling the air approximately 8.degree. F. by the heat pipe precooling coil prior to introduction to the evaporator coil; this reduces the load slightly on the evaporator coil and allows that coil to cool the air approximately 2.degree. F. more than normal in an effort to remove more moisture from the air and provide better dehumidification of the space. The air leaving the evaporator coil is then reheated by the heat pipe reheat coil by the same amount of precooling, approximately 8.degree. F., so that the air is not too cold when introduced into the space being cooled.
The particular approach disclosed by Dinh may require additional system modifications, however. Because the air being introduced to the space is now warmer due to the effect of the heat pipe, the supply air fan and duct system may require upgrading to increase the supply air flow and maintain the design space temperature.
The efficiency gains produced by the Dinh system from the precooling may be offset by the increased cooling work performed by the evaporator coil in cooling the air to lower temperatures. Additionally, the warmer discharge air temperature caused by the heat pipe reheat coil may result in a larger air flow requirement to handle the sensible cooling load, possibly causing an increase in the fan horsepower requirement. The heat pipes also add static pressure loss to the system that the fan must overcome, again possibly increasing the fan horsepower requirement.
The approach disclosed by Dinh locates the heat pipe as the first coil in the entering air stream of the air conditioning system and thus exposes it to widely varying entering air conditions as the seasons and temperatures change, causing variations in the heat pipe performance. For example, at part load conditions the air entering the first coil of the heat pipe may now be 68.degree.-70.degree. F. or possibly even colder in the winter instead of the 78.degree.-80.degree. F. designed for in the summer. Under these new conditions, the heat pipe may not provide as much precooling and reheating as would be desired, and may possibly reduce the overall dehumidification below that required.
Another type of system that has been used in the Melbourne, Fla. area for several years is a two-stage refrigeration device, called a "latent machine." This design uses two stages of a conventional vapor compression refrigeration cycle with heat recovery to cool the room air to a temperature low enough to maintain the desired humidity control in the space and reheats the air back to room temperature. The process embodied in this design is to cool the room air to 52.degree. F. saturated conditions in the first stage of refrigeration. The heat is rejected to the outdoors via a standard condenser arrangement, and the air is then further cooled to 37.degree. F. saturated conditions in the second stage of refrigeration, then reheated to 75.degree. F. dry bulb, 55.degree. F. wet bulb using a conventional condenser coil in the unit supply air stream that transports the heat of rejection from the second stage of refrigeration into the supply air. The air is thus reintroduced to the room as dehumidified, room temperature air that provides latent cooling only.
This "latent machine" system has proven to be excellent for humidity control and is very efficient on return air systems. It does, however, sacrifice some efficiency on outside air applications in that large amounts of energy are required when used with a supply of warmer and more humid air from the outside.
Another previous system, known as the "Tricoil System" of The Sensible Equipment Company of Orlando, Fla., utilizes a water-based coil "runaround loop." In this system, a water-cooled precooling coil is placed in the entering air prior to an evaporator coil, then a water-heated reheat coil is placed in the air leaving the evaporator coil, with the water recirculated between the precool and reheat coils with a small pump, in effect providing a heat transfer mechanism similar to the heat pipe. This system has the same advantages of the Dinh system with regard to humidity control and system efficiency and can again be a very simple and effective retrofit method for an oversized room air conditioner. The system does allow for a variable amount of reheat to be added to the system for part load humidity control and requires a separate heat source to introduce a false load on the air conditioning system so as to maintain proper dehumidification with some increased operating costs.
It is therefore a desirable objective to provide control of room humidity without the use of reheat, without overcooling the space, and without sacrificing the efficiency of the air conditioning system.
It is also desirable to provide control of the room humidity regardless of the temperature in the space or outside and regardless of the solar load.
Another desirable objective would be to provide adequate outside air for ventilation to benefit occupant health and maintain lower levels of airborne pollutants and fresher air in the building per the latest standards of the air conditioning industry, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Standard 62-1989.
It is also a desirable objective to provide all of these aspects in one system: proper control over the supply and cooling of outside air ventilation, space dehumidification, and high efficiency. Such control would ensure that the building would be maintained in proper operating condition at all times.
It was in an effort to achieve these objectives that the present invention has evolved.